Components subject to conditions such as mechanical stress and elevated temperature are known to experience dimensional changes over a lifetime of use. For example, in the turbine section of a gas turbine engine, a rotating blade rotates at high speeds. Inertia of the airfoil portion causes mechanical stress (tension) in the span of the airfoil (from a base of the airfoil to a tip of the airfoil). This span-wise mechanical stress may cause a length of the span to increase (e.g. creep) over the life of the airfoil. In addition, it is known that a material's properties are temperature dependent, and an ability of a material to resist this creep decreases with an increase in temperature. Since turbine airfoils rotate within a flow of combustion gases at a very high temperature, the elevated temperatures contribute to the airfoil creep.
Surrounding a stage of rotating blades in a gas turbine engine is a blade ring that may include an abradable surface. This abradable surface is disposed immediately radially outward of tips of the blades. The abradable surface and the blade tips cooperatively form a seal intended to direct the combustions gases across the sides of the airfoils and not over the tip of the airfoils. Accordingly, a gap between the abradable surface and the tips of the airfoils must be controlled. It is known that periodic thermal transients may cause relative thermal growth such that the airfoil tips engage the abradable material. The abradable material is selected such that the blade tips survive by abrading the abradable material. However, this situation is not desirable and eventually may require the blade ring to be replaced due to the degraded seal.
Consequently, like many components within the gas turbine engine, the blade airfoils are constrained to operate within a certain dimensional range. For these components, this requirement exists despite the operating conditions that contribute to the change in span length of the component. Therefore, there exists a need to monitor components so that any dimensional changes can be determined and problems associated with an increase in their dimension can be avoided.